The present invention relates to a procatalyst, a procatalyst precursor, and a process for the preparation of a multimodal ethylene homopolymer and/or ethylene/1-olefin copolymer by gas-phase polymerisation. More particularly, the present invention relates to a procatalyst, a procatalyst precursor, and the preparation of a bimodal ethylene polymer by gas-phase polymerisation in two steps, wherein an ethylene polymer fraction with a low melt flow rate is preferably prepared in the first step, and an ethylene polymer fraction with a high melt flow rate is preferably prepared in the second step.
When talk is about the "modality" of a polymer, reference is made to the form of its molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight. If the polymer is produced in a sequential step process, utilizing reactors coupled in series and using different conditions in each reactor, the different fractions produced in the different reactors will each have their own molecular weight distribution. When the molecular weight distribution curves from these fractions are superimposed into the molecular weight distribution curve for the total resulting polymer product, that curve will show two or more maxima or at least be distinctly broadened in comparison with the curves for the individual fractions. Such a polymer product, produced in two or more serial steps, is called bimodal or multimodal depending on the number of steps. In the following all polymers thus produced in two or more sequential steps are called "multimodal". It is to be noted here that also the chemical compositions of the different fractions may be different. Thus one or more fractions may consist of an ethylene copolymer, while one or more others may consist of ethylene homopolymer.
The "melt flow rate", (MFR), of a polymer is determined according to ISO 1133 and is often (erroneously) referred to as the "melt index". The MFR, which is measured in g/10 min of polymer discharge under specified temperature, pressure and die conditions, is a measure of the viscosity of the polymer, which in turn for each polymer chemical type is mainly influenced by its molecular weight distribution, but also by its degree of branching etc. For a specific type of polymer the higher the value of its MFR, the lower is its mean molecular weight. When used herein, the expressions "low melt flow rate" or "low MFR" imply an MFR.sub.2.16 (determined according to ISO 1133, condition 7) of about 0.001-10.0 g/10 min. Similarly, by the expression "high melt flow rate" or "high MFR" is implied an MFR.sub.2.16 (determined according to ISO 1133, condition 4) of about 0.1-5000 g/10 min. These ranges overlap one another to a small extent, but it is to be understood that for a multimodal polymer produced in a two-step process, the polymer fraction produced in the "low MFR" step always has a lower MFR than the polymer fraction produced in the "high MFR" step.
By the term "halogen" represented by "X" in the formulas and used in this specification and claims, e.g. in the expressions "a halogen containing compound" or "a halogenating agent", is meant compounds containing a halogen selected from Cl, Br, I, F preferably from Cl or Br, and most preferably Cl.
It is known to prepare multimodal olefin polymers, preferably multimodal ethylene polymers, in two or more reactors connected in series. As examples of this prior art, EP 369 436 and EP 503 791 may be mentioned.
EP 369 436 relates to an optimised process for in situ blending of polymers, such as the preparation of a bimodal polyethylene polymer in order to provide polymers having desirable properties and an enhanced processability, in particular extrudability. In the process, a mixture of ethylene and at least one .alpha.-olefin having 3-10 carbon atoms is contacted, in at least two fluidised-bed reactors connected in series, with a catalyst comprising: a complex of magnesium, titanium, a halogen and an electron donor; at least one activator compound; and a hydrocarbyl aluminum cocatalyst. The complex may be prepared by using e.g. MgCl.sub.2 as a magnesium compound, TiCl.sub.4 as a titanium compound, and tetrahydrofuran as an electron donor. Examples of preferred activator compounds are triethylaluminium, triisobutylaluminium, and diethylaluminium chloride. Examples of preferred cocatalysts are triethylaluminium and triisobutylaluminium. The catalyst is preferably provided on a support, such as silica. In the process, a copolymer having a low melt index (measured according to ASTM D-1238, condition E) of about 0.001-1.0 g/10 min is prepared in the first reactor, and a copolymer having a high melt index of about 0.1-1000 g/10 min is prepared in the second reactor. The order of preparation may be reversed. In the reactor where the copolymer having a low melt index is prepared, hydrogen is optionally present in an amount of about 0.001-0.5 mole of hydrogen per mole of ethylene and .alpha.-olefin, while hydrogen is present in an amount of about 0.05-3.5 mole of hydrogen per mole of ethylene and .alpha.-olefin in the reactor where the high melt index copolymer is prepared. Cocatalyst is introduced into each reactor to restore the activity of the catalyst.
EP 503 791 relates to a process for producing bimodal ethylene polymers by gas-phase polymerisation using two gas-phase, fluidised-bed reactors connected in series. In the first reactor (step 1), an ethylene polymer of high molecular weight is produced, and in the other reactor (step 2), an ethylene polymer of low molecular weight is produced. The catalyst used for the polymerisation is a Ziegler-Natta catalyst of the same type as is used in EP 369 436, i.e. it comprises a complex of magnesium, titanium, halogen and an electron donor, together with a cocatalyst of hydrocarbyl aluminum, and the catalyst is preferably supported on a carrier, such as silica. In the polymerisation, the molar ratio of hydrogen to ethylene is controlled so as to be at least 8 times higher in the second reactor. According to EP 503 791, the problem is that the productivity in the second reactor is reduced, and this problem is, according to EP 503 791, solved by increasing the temperature in the second reactor; adding a cocatalyst to the second reactor; making the ethylene partial pressure in the second reactor at least 1.7 times higher than that in the first reactor; and preferably prolonging the residence time in the second reactor so that it is, for instance, twice as long as that in the first reactor.
The prior-art technique, as disclosed in EP 369 438 and EP 503 791, for the preparation of multimodal polyethylene by gas-phase polymerisation suffers from a number of drawbacks. Thus, procatalyst and cocatalyst have to be added, not only to the first reactor, but at least cocatalyst must normally be added also to the subsequent reactors to restore the activity of the catalyst. Further, the decreased activity of the catalyst after the first reactor is compensated for by increasing the ethylene partial pressure in the subsequent reactor(s). Yet another measure intended to compensate for the decreased activity of the catalyst is to prolong the residence time of the reaction mixture in the subsequent reactor(s). Finally, it is often necessary to use a different, larger reactor in the second step and any subsequent steps in order to prolong the residence time. These measures, which basically are necessitated by the poor balance of activity of the catalysts in the different reaction steps, and more specifically by their low activity in the presence of more substantial amounts of hydrogen, entail a more complicated control of the polymerisation parameters, higher production costs and higher investment costs. Accordingly, there is a need for a catalyst which has a good basic activity and furthermore retains a similar level of activity in each reaction step, also in the presence of hydrogen, i.e. shows a good activity balance.